Type I diabetes is a widespread metabolic disorder caused by failure of beta cells of the pancreas to secrete sufficient insulin. Insulin is required for the uptake of glucose in most cell types, and inadequate insulin production causes reduced glucose uptake and elevated blood glucose levels. Without proper treatment, diabetes can be fatal. Treatment with insulin, while life-saving, often does not provide sufficient control of blood glucose to prevent the life-shortening complications of the disease, and this has given rise to intensive research into better methods of achieving and sustaining normoglycemia. Among the newer treatment strategies that have been proposed, transplantation of pancreatic beta islet cells, obtained either from other humans or animals, has received the significant attention worldwide. This is because islet cell transplantation can restore not only the insulin-secreting unit, but also the precise fine-tuning of insulin release in response to multiple neural and humoral signals arising within and beyond the islets of Langerhans.
Ever since the first experimental attempts to ameliorate Type I diabetes by transplantation of allograft donor islets the field has been challenged by the need for improved methods of retrieving and/or obtaining islets from donor pancreata. There is a considerable worldwide effort to further develop the concept for treating Type I diabetes by transplanting islets, but clinical application of the techniques developed in animal models is fraught with many challenges.
The source of the islets remains a primary concern, and isolation from donor pancreases demands resolution of questions concerning the source, supply, and condition of the donor organs. Reliance upon an adequate supply of human organs for this purpose is considered futile, such that alternative sources are actively been sought (Bonner-Weir, S. et al., New sources of pancreatic beta-cells, Nat. Biotechnol. 23:857-861, 2005; Hering, B. S. et al., New sources of pancreatic beta-cells, Nat. Biotechnol. 23:857-861, 2005; Hering, B. J. et al., Prolonged diabetes reversal after intraportal xenotransplantation of wild-type porcine islets in immunosuppressed nonhuman primates, Nat. Med., 12:301-303, 2006; Inada, A.; Bonner-Weir, S. et al., How can we get more beta cells?, Curr. Diab. Rep., 6:96-101, 2006).
Various mammals are considered optimal candidates for xenogeneic islet transplantation. Of the potential mammals, pigs are considered the donor species of choice for xenogeneic islet transplantation for a number of compelling reasons. Pigs share many physiological similarities to humans and porcine insulin has demonstrated clinical efficacy for many years. Pigs are raised as a food source and provide an ethical source of donor islets by being housed in a controlled environment to ensure safety for porcine islet xenotransplantation. However, experiences in many laboratories over the past 10 years show that isolation of porcine islets appears to be more difficult (Finke, E., et al., Large scale isolation, function, and transplantation of islets of Langerhans from the adult pig pancreas. Transplant. Proc. 23:772-773, 1991; Giannarelli, R. et al., Preparation of pure, viable porcine and bovine islets by a simple method. Transplant. Proc., 26:630-631, 1994; Marchetti, P. et al., Automated largescale isolation, in vitro function and xenotransplantation of porcine islets of Langerhans, Transplantation 52:209-213, 1991; O'Neil, J. J. et al., The isolation and function of porcine islets from market weight pigs. Cell Transplant., 10:235-246, 2001; Toso, C. et al., Isolation of adult porcine islets of Langerhans. Cell Transplant., 9:297-305, 2000), compared with the isolation of human (Kenmochi, T. et al., Improved quality and yield of islets isolated from human pancreas using two-step digestion method, Pancreas 20:184-190, 2000), bovine (Figliuzzi, M. et al., Influence of donor age on bovine pancreatic islet isolation, Transplantation, 70:1032-1037, 2000), or rodent islets (Shapiro, A. M. et al., High yield of rodent islets with intraductal collagenase and stationary digestion—a comparison with standard technique, Cell Transplant., 5:631-638, 1996).
For example, porcine islets are less compact and tend to fragment during the isolation procedure and during prolonged periods of in vitro culture (Ricordi, C. et al., A method for the mass isolation of islets from the adult pig pancreas, Diabetes, 35:649-653, 1986). Moreover, the age, and even the strain, of the donor pig has been documented by several groups to markedly influence the islet isolation process, with young, so-called market size pigs (<6 months old) proving to be particularly difficult as a source of transplantable islets (Bottino, R. et al., Isolation outcome and functional characteristics of young and adult pig pancreatic islets for transplantation studies, Xenotransplantation, 14:74-82, 2007; Dufrane, D. et al., Impact of porcine islet size on cellular structure and engraftment after transplantation: Adult versus young pigs, Pancreas 30:138-147, 2005; Toso, C. et al., Isolation of adult porcine islets of Langerhans. Cell Transplant., 9:297-305, 2000). Islets from adult pigs (>2 years old) offered not only higher yields, but retained the ability to preserve intact morphology during the isolation process and culture, in association with higher functional properties after transplantation. Despite the challenge encountered by many groups attempting to isolate islets from young pigs, donor pigs of market weight (<80 kg=<12 months old) are preferred to retired breeders (>200 kg=>2 years old) due to their abundance, lower animal and husbandry costs, and they are more suitable to meet regulatory guidelines for donor tissue for xenotransplantation. If the supply of islet cells (including but not necessarily limited to beta cells) could be augmented by culturing the donated islets from more readily available sources, which may be less compact and may or may not display a propensity for fragmenting (such as islets obtained from young pigs), in cell culture, such a new source of islets may provide sufficient material to allow a new treatment for insulin-dependent diabetes.
However, despite many efforts to improve the technique of islet isolation and preparation, the field remains constrained by the limitations and vagaries of enzymatic digestion of a gland that comprises less than 5% endocrine tissue. Consequently, harvesting islets from a single donor pancreas often yields insufficient islet mass to reverse diabetes in a recipient, such that multiple donors often have to be considered for treating a single recipient. The potential for xenotransplantation to relieve the demand on an inadequate supply of human pancreases depends upon improvements in efficacy and efficiency of techniques for isolating and preparing islets from the source pancreases (Hering, B. J. et al., The International Xenotransplantation Association consensus statement on conditions for undertaking clinical trials of porcine islet products in type 1 diabetes—executive summary, Xenotransplantation 16:196-202, 2009).